A description is provided with reference to an electrophotographic image forming apparatus, such as a laser beam printer shown in FIG. 2.
Referring to FIG. 2, the electrophotographic image forming apparatus forms an electrostatic latent image by irradiating an electrophotographic image bearing member 1, uniformly charged by a charging means 2, with light corresponding to image information, and makes visible the electrostatic latent image as an image by supplying developer (hereinafter, referred to as “toner”) as a recording material by the use of a developing means. Further, the toner image is transferred from the image bearing member 1 onto a recording paper P as a recording medium, and the recording paper P holding the toner is sent to a fixing device 18 so as not to disturb the toner image, which image is then subjected to fixation under heat and pressure by the fixing device 18 to be recorded and outputted as a permanent image on the recording paper P. To the developing means, a toner container as a developer containing portion 4 containing the toner is connected. The toner is consumed by forming the image. In many cases, the toner container, the developing means, the image bearing member, the charging means and so on, are integrally constituted as a process cartridge (hereinafter, referred to as a “cartridge”). When the toner is consumed, a user can form again an image by replacing the cartridge with a new one.
In the cartridge, a predetermined amount of toner determined by a container volume is contained. Accordingly, the number of printable sheets by the user generally correlates with the amount of toner. Users who save toner by reducing toner consumption to permit a larger number of printable sheets are increasing. In addition, laser beam printers having an image formation mode, such as a low (toner) consumption mode or a draft mode, capable of automatically decreasing the amount of toner consumption are also increasing.
As a means for decreasing the toner consumption amount, it is possible to use a means for changing a developing contrast, a means for changing a laser light quantity, etc. By changing the developing contrast or the laser light source, a latent image formed on the image bearing member is changed. As a result, toner coverage can be reduced at the time of development.
However, in the case where the amount of toner consumption is decreased only by the developing contrast or the laser light quantity, a thin line image or a character image has a very narrow line width to provide a poor image quality in some cases even under such a condition that a change in image quality is less conspicuous with respect to a solid black image having a large area to some extent.
For this reason, as a means for reducing the toner consumption amount while ensuring an adequate line width, a control method has been developed to print an image frame portion constituted by a binary image at an original density but the amount of toner consumption is decreased at an inner portion of the image, to permit a decrease in the toner consumption amount while ensuring an adequate line width (e.g., Japanese Laid-Open Patent Application No. Hei 9-085993). More specifically, as shown in FIG. 3, the control method effects such an image processing that an original image (image data) 301 to be printed is changed into a dither image 302, wherein a frame portion, as a concentrated pixel area like a solid black image, is printed at an original density, but an inner portion is provided with distributed blank dots which are not printed or a halftone image 303, wherein the amount of emission of a laser or a laser-on period is changed on a one dot unit basis.
Herein, such an image formation mode for suppressing a toner coverage by modulating an image is referred to as a “low (toner) consumption mode”.
However, the above-described conventional image control means is accompanied with the following problems.
In the low consumption mode image control means which has been conventionally used, as described above, a frame portion of a concentrated pixel portion of a resultant image is printed at an original density and the image is converted into a dither image and a halftone image at an inner portion (central portion) to reduce the amount of toner consumption. In this case, the image control means is uniformly adapted to all the images except for those at the frame portion. The proportion between a pattern of the dither image and a pattern of the halftone image is switched according to the use circumstances, whereby it becomes possible to provide a low consumption mode which maintains image quality.
However, in the case of effecting a low consumption mode with the dither image, when the amount of toner consumption is intended to be further decreased compared with that in the conventional low consumption mode, there arises such a problem that a blank dot portion is very conspicuous to make an image, to be originally a solid black image, a mesh image.
Further, in recent laser beam printers, the number of available sheets for a toner cartridge is increased with its popularization, so that a further increase in life and the number of available sheets has been practiced. However, the long life of the toner cartridge leads to a difference in density or line width of a solid black image between a toner cartridge in an initial state and a toner cartridge which has been subjected to printing (copying) of a very large number of sheets. As a result, the resultant image quality deteriorates in some cases. Particularly, this phenomenon is more noticeable in the case where a material which is readily abraded in continuous image formation is used in a photosensitive layer of the image bearing member (or in the case where a material having a different sensitivity characteristic of the image bearing member is used).
Further, in the case of effecting a low consumption mode with the halftone image obtained by changing an emission time or an emission light quantity of a laser scanner, there arises such a problem that the low consumption mode is more liable to be affected by a durability change of a photosensitive layer of the image bearing member. More specifically, with respect to an ordinary laser light in the case where the halftone treatment is not performed, there is substantially no influence by a sensitivity change due to abrasion of the photosensitive layer caused by long-term use of the image bearing member. However, with respect to a laser light changed in emission time or emission light quantity, the sensitivity of the image bearing member becomes lower as the photosensitive layer becomes thinner by a durability deterioration of the photosensitive layer, i.e., abrasion of the image bearing member. As a result, a large density lowering and a deterioration in line width are caused to occur.
Further, it is possible to mount a density sensor for detecting a sensitivity change of the image bearing member or an exposure potential sensor for the image bearing member but the mounting of the sensors is accompanied by the problem of cost for incorporating detection circuits for the above-described sensors and the problem of ensuring mounting space for mounting the sensors.
In addition, in the above-described pattern difference in the area of the image, such as the solid black image or line width, as in the conventional image control means, the amount of toner consumption required to maintain the quality of an image area limits the decrease in the toner consumption amount, so that it is necessary to sacrifice a decreased degree of the toner consumption amount if the toner consumption amount decreasing means uniformly decreases the amount of toner consumption irrespective of image area.
Further, with diversification of cartridges, a cartridge which can be detachably mountable to the identical image forming apparatus but has a different toner volume, has been put into practical use. For example, a cartridge that achieves a low price by reducing toner volume, has been commercialized. An image bearing member (photosensitive member) in such a cartridge having a different toner volume is designed to have a thickness adapted for respective toner volumes so as to reduce costs in some cases. For this reason, in the case where the toner consumption amount is decreased by using the low toner consumption mode, an appropriate initial drum thickness is different depending on the respective cartridges. As a result, there arises such a problem that the progression of durability of drum use or a difference in image quality becomes large.